1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for fabricating the liquid crystal display device. More particularly, the present invention relates to a liquid crystal display device having a transmission display region and a reflection display region in each pixel, and a method for fabricating such a liquid crystal display device.
2. Description of the Related Art
Due to the features of being thin and consuming low power, liquid crystal display devices have been used in a broad range of fields including office automation (OA) apparatuses such as wordprocessors and personal computers, portable information apparatuses such as portable electronic schedulers, and a camera-incorporated VCR provided with a liquid crystal monitor.
Such liquid crystal display devices include a liquid crystal display panel which does not emit light itself, unlike a CRT display and an electroluminescence (EL) display. Therefore, a so-called transmission type is often used as the liquid crystal display device, which includes an illuminator called a backlight disposed at the rear or one side thereof, so that the amount of the light from the backlight which passes through the liquid crystal panel is controlled by the liquid crystal panel in order to realize image display.
In such a transmission type liquid crystal display device, however, the backlight consumes 50% or more of the total power consumed by the liquid crystal display device. Providing the backlight therefore increases the power consumption.
In order to overcome the above problem, a reflection type liquid crystal display device has been used for portable information apparatuses which are often used outdoors or carried with the users. Such a reflection type liquid crystal display device is provided with a reflector formed on one of a pair of substrates in place of the backlight so that ambient light is reflected from the surface of the reflector.
Such a reflection type liquid crystal display device is operated in a display mode using a polarizing plate, such as a twisted nematic (TN) mode and a super twisted nematic (STN) mode which have been broadly used in the transmission type liquid crystal display devices. In recent years, there has been vigorous development of a phase change type guest-host mode which does not use a polarizing plate and thus realizes a brighter display.
The reflection type liquid crystal display device using the reflection of ambient light is disadvantageous in that the visibility of the display is extremely lower when the surrounding environment is dark. Conversely, the transmission type liquid crystal display device is disadvantageous when the environment is bright. That is, the color reproducibility is lower and the display is not sufficiently recognizable because the display light is less bright than the ambient light. In order to improve the display quality under a bright environment, the intensity of the light from the backlight needs to be increased. This increases the power consumption of the backlight and thus the resultant liquid crystal display device. Moreover, when the liquid crystal display device needs to be viewed at a position exposed to direct sunlight or direct illumination light, the display quality is inevitably lower due to the ambient light. For example, when a liquid crystal display screen fixed in a car or a display screen of a personal computer used at a fixed position receives direct sunlight or illumination light, surrounding images are mirrored, making it difficult to observe the display itself.
In order to overcome the above problems, a construction which realizes both a transmission mode display and a reflection mode display in one liquid crystal display device has been disclosed in, for example, Japanese Laid-Open Publication No. 7-333598. Such a liquid crystal display device uses a semi-transmissive reflection film which transmits part of light and reflects part of light.
FIG. 52 shows such a liquid crystal display device using a semi-transmissive reflection film. The liquid crystal display device includes polarizing plates 30a and 30b, aphase plate 31, a transparent substrate 32, black masks 33, a counter electrode 34, alignment films 35, a liquid crystal layer 36, metal-insulator-metal (MIM) elements 37, pixel electrodes 38, a light source 39, and a reflection film 40.
The pixel electrodes 38, which are the semi-transmissive reflection films, are extremely thin layers made of metal particles or layers having sporadical minute hole defects or concave defects therein formed over respective pixels. Pixel electrodes with this construction transmit light from the light source 39 and at the same time reflect light from outside such as natural light and indoor illumination light, so that both the transmission display function and the reflection display function are simultaneously realized.
The conventional liquid crystal display device shown in FIG. 52 has following problems. First, when an extremely thin layer of deposited metal particles is used as the semi-transmissive reflection film of each pixel, since the metal particles have a large absorption coefficient, the internal absorption of incident light is large and some of the light is absorbed without being used for display, thereby lowering the light utilization efficiency.
When a film having sporadical minute hole defects or concave defects therein is used as the pixel electrode 38 of each pixel, the structure of the film is too complicated to be easily controlled, requiring precise design conditions. Thus, it is difficult to fabricate the film having uniform characteristics. In other words, the reproducibility of the electrical or optical characteristics is so poor that control of the display quality in the above liquid crystal display device is extremely difficult.
For example, if thin film transistors (TFTS), which in recent years have been generally used as the switching elements of liquid crystal display devices, are attempted to be used for the above liquid crystal display device shown in FIG. 52, an electrode for the formation of a storage capacitor in each pixel needs to be formed by an electrode/interconnect material other than that for the pixel electrode. In this case, the pixel electrode made of the semi-transmissive reflection film, as in this conventional device, is not suitable for the formation of a storage capacitor. Moreover, even when the semi-transmissive reflection film as the pixel electrode is formed over part of the interconnects and elements via an insulating layer, the pixel electrode which includes a transmissive component hardly contributes to an increase in the numerical aperture. Also, if light is incident on a semiconductor layer of the switching element such as a MIM and a TFT, an optically pumped current is generated. The formation of the semi-transmissive reflection film as the light-shading layer is insufficient for the protection of the switching element from light. To ensure light-shading, another light-shading film is required to be disposed on the counter substrate.